The field of the invention is in the metal-gas cell and battery art.
The description of the prior art may best be understood by considering a specific example. In a typical, current state of the art, cylindrical 50 ampere-hour nickel-hydrogen cell, the leads which carry the electrical current to and from the plates in the stack to the terminals will be longer for those plates near the bottom of the stack (i.e., those farthest from the terminals) than those leads from the plates near the top of the stack (see FIG. 1). As a result of the different lead lengths, the electrical resistance of the leads going to the lower plates is considerably greater than those of the shorter leads going to the upper plates. The effect of the different resistances is to cause the plates at the top of the stack to deliver more current than the bottom plates during discharge and to receive more current during charge. For example, in a typical manufactured 50 amp-hr Ni-H cell the current imbalance is .+-. 40% from top to bottom based upon average current. This is quite detrimental to efficient operation of the cell and to cell life. Normally cells are run to a nominal depth of discharge of the whole cell of 80%. This subjects the upper plates to approximately a 100% depth of discharge substantially shortening their life.
An attempt to overcome this variation in lead resistance has been to make the leads of sufficient cross sectional area that the voltage drop in the leads is very low and hence the difference in voltage drop between the long leads and the short leads becomes an insignificantly small value and all the plates substantially discharge and charge the same amount. However, it has been found that to reduce the imbalance to .+-. 15% that the weight of the leads (for the conventional 50 amp-hr Ni-H cell being described) would be increased 0.12 pounds, and it is very desirable to reduce the percent further to prolong cell cycle life. This increase in weight imposes a large weight penalty on the cell for the majority of its intended useage (space borne devices) and makes it generally unacceptable. Another attempt to cope with this problem has been to make all lead lengths the same, thus all leads would have the same resistance (and the same voltage drop). The excess lead lengths for the shorter distances, i.e., the upper plates, are merely doubled up or folded back on themselves in the upper cavity of the pressure vessel. This solution is likewise generally unacceptable due to the weight penalty involved being substantially as serious as in the previous attempted solution.
The conventional problems of resistance and current carrying capabilities of leads and connections between the plates of storage cells and to the battery terminals are generally old and well known and in general, they have been adequately solved. Examples of such are contained in U.S. Pat. Nos. 2,515,204; 2,677,713; and 3,650,833 to patentees L. Evans, V. Weil et al, and E. Sandberg, respectively. The problems encountered with the relatively new "exotic" batteries and the restrictions placed upon them in their usage in outer space environments are novel. It is in the critical and extremely important areas of weight and cell life with which this invention further advances the art.